In recent years, electroacoustic transducers such as a speaker and a buzzer using a piezoelectric effect are widely used. For example, the following Patent Document 1 discloses a sounder 101 illustrated in an elevational cross-sectional view in FIG. 11. The sounder 101 includes a box 102. One end portion of a piezoelectric vibrating element 103 is coupled to the inner wall of the box 102. The piezoelectric vibrating element 103 has a configuration in which electrodes 103a and 103b are provided on opposite surfaces of a piezoelectric ceramic plate. By applying an alternating electrical field to the piezoelectric ceramic plate from the electrodes 103a and 103b, the piezoelectric ceramic plate is polarized so that vibration thereof is excited.
In the sounder 101, the piezoelectric vibrating element 103 is supported at one end portion thereof, and the other end portion of the piezoelectric vibrating element 103 is a free end portion. That is, since the piezoelectric vibrating element 103 is supported in the cantilever manner, the free end portion of the piezoelectric vibrating element 103 can be significantly displaced. Accordingly, a high sound pressure can be obtained.
On the other hand, the following Patent Document 2 discloses a piezoelectric ceramic speaker illustrated in FIG. 12. In a piezoelectric ceramic speaker 111, one end portion of a piezoelectric vibrating element 113 is coupled to a frame member 112. The piezoelectric vibrating element 113 is supported at one end portion thereof in the cantilever manner, and the other end portion thereof is a free end portion. On the side of the free end portion, the center of a cone-shaped diaphragm 114 is fixed to the piezoelectric vibrating element 113. Accordingly, at the time of bending vibration of the piezoelectric vibrating element 113, the cone-shaped diaphragm 114 coupled to the free end portion of the piezoelectric vibrating element 113 vibrates. As a result, a high sound pressure can be obtained.
Patent Document 1: Japanese Unexamined Utility Model Application Publication No. 63-191800
Patent Document 2: Japanese Utility Model Registration No. 3068450
In the sounder 101, the peripheral portion of the piezoelectric vibrating element 103 excluding the portion coupled to the box 102 is exposed in the box 102. Accordingly, when the piezoelectric vibrating element 103 vibrates, the air pressure on the side of one surface of the piezoelectric vibrating element 103 and the air pressure on the side of the other surface of the piezoelectric vibrating element 103 cancel each other. As a result, sound in a low frequency range cannot be obtained. That is, a high sound pressure cannot be obtained over a wide range of frequencies.
On the other hand, in the piezoelectric ceramic speaker 111 disclosed in Patent Document 2, the cone-shaped diaphragm 114 capable of directly acting on the air generates an acoustic wave. Accordingly, in addition to the piezoelectric vibrating element 113, the large cone-shaped diaphragm 114 is required. As a result, the piezoelectric ceramic speaker 111 becomes larger in size. This makes it difficult to reduce the thickness of the piezoelectric ceramic speaker 111. Furthermore, the number of components is increased, and the number of manufacturing processes is therefore increased. This leads to the increase in cost.
Furthermore, natural in-plane vibration of the cone-shaped diaphragm 114 occurs. As a result, a frequency characteristic becomes deteriorated. Still furthermore, although the piezoelectric vibrating element 113 is supported at one end portion thereof in the cantilever manner, the free end portion thereof is coupled to the cone-shaped diaphragm 114. Accordingly, there is a vibration mode in which the free end portion of the piezoelectric vibrating element 113 is hardly displaced. As a result, a sharp dip occurs in a frequency characteristic.